Question

即使实例化了我的活动按钮，我的应用程序仍然崩溃。我究竟做错了什么？这是我的xml和Java代码。请帮助我，我现在真的需要帮助。我不知道该怎么办。这是我的错误堆栈：

Process: com.example.dith.finalproject3, PID: 9301
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.dith.finalproject3/com.example.dith.finalproject3.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setVisibility(int)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2913)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setVisibility(int)' on a null object reference
        at com.example.dith.finalproject3.MainActivity.expansions(MainActivity.java:193)
        at com.example.dith.finalproject3.MainActivity.onCreate(MainActivity.java:63)
        at android.app.Activity.performCreate(Activity.java:7136)
        at android.app.Activity.performCreate(Activity.java:7127)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1271)

这是我的Java代码：

public void expansions()
{
        btnCalendar = (ImageButton) findViewById(R.id.btnCalendar);
        btnCalendarHide = (Button) findViewById(R.id.btnCalendarHide);
        expCalendar = (View) findViewById(R.id.expCalendar);
        expCalendar.setVisibility(View.INVISIBLE);
}

下面写的是我的XML文件：

<LinearLayout
                        android:id="@+id/expCalendar"
                        android:layout_width="match_parent"
                        android:layout_height="wrap_content"
                        android:orientation="vertical">

                        <TextView
                            android:id="@+id/txtCalendar"
                            android:layout_width="wrap_content"
                            android:layout_height="wrap_content"
                            android:padding="@dimen/spacing_large"
                            android:text="Calendar Placeholder" />

                        <View
                            android:layout_width="match_parent"
                            android:layout_height="1dp"
                            android:background="@color/grey_10" />

                        <LinearLayout
                            android:layout_width="match_parent"
                            android:layout_height="wrap_content"
                            android:gravity="end"
                            android:orientation="horizontal">

                            <Button
                                android:id="@+id/btnCalendarHide"
                                style="@style/Widget.AppCompat.Button.Borderless"
                                android:layout_width="wrap_content"
                                android:layout_height="wrap_content"
                                android:gravity="center"
                                android:text="HIDE" />

                        </LinearLayout>

非常感谢您的快乐！

Answer 1

您的问题在这里：

@WebFilter(filterName = "Filter", urlPatterns = { "/filter/*" })
public class WebFilter implements Filter {
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
         .......
    }

   .......
  }

expCalendar = (View) findViewById(R.id.expCalendar); expCalendar.setVisibility(View.INVISIBLE);方法返回findViewById。这意味着null是expCalendar，这意味着您试图在不存在的对象上调用null方法。直接的答案是在使用之前检查setVisibility是否为空：

expCalendar

但是，这并没有解决隐含的问题，即您是否担心根本找不到expCalendar = (View) findViewById(R.id.expCalendar); if (expCalendar != null){ expCalendar.setVisibility(View.INVISIBLE); }。不幸的是，我对Android的了解不足，无法为您提供帮助。

原因：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法'void android.view.View.setVisibility（int）'

1 个答案: